The present invention relates to a method for measuring a sample in a liquid scintillation counter where the sample to be analysed is mixed with a liquid or a solid scintillator and is measured with two photo detectors, e.g. photo multiplier tubes or the like, the detectors working in coincidence and situated on the opposite sides of the sample.
As is well known, the liquid scintillation counting method is commonly used for measuring radioactive isotopes, e.g. tritium and carbon-14, which emit low-energy beta or corresponding particles. The range of emission of the low-energy beta particle in the sample is generally few tens of micrometers at the most. Consequently, the sample to be analysed has to be placed in direct contact with the scintillation medium, liquid or solid, either by dissolving it into scintillation molecules of the medium or attaching it to a scintillation particle using some special binding reactions. In this interaction process most of the kinetic energy of the interacting beta particle is absorbed by the medium or solvent and then transferred to the scintillator or solute that emits scintillation photons, whose amount is proportional to the energy of the interacted beta particle.
These scintillation photons are detected usually by two photo multiplier tubes, working in coincidence, that convert the photons into electric pulses. The coincidence method eliminates the thermal noise of the photo multiplier tubes. The heights of the pulses from the sample are proportional to the amount of emitted scintillation photons and thus proportional to the energy of the interacted beta particle. Normally, the pulses from both the photo multiplier tubes are summed together.
Because the energies of the emitted beta particles are distributed in a way characteristic to the beta decay of the isotope to be counted, a continuous spectrum corresponding to the energy distribution of the emitted beta particles is obtained by means of the multichannel analyser incorporated in the counter. This continuous spectrum has certain characteristic properties, e.g. total counts, number of counts in a certain "counting window" or channel range of the multichannel analyser, end point, maximum value and center of mass, i.e. the centroid of the obtained spectrum. The channel of the multichannel analyser can be calculated, in which the end point, the maximum value and the center of the mass are located, i.e. the channel co-ordinates of these values can be determined. The channel co-ordinate of the center of the mass of the sum spectrum is generally used as a measure of the quench level of the sample.
The counting efficiency of a liquid scintillation counter means the efficiency of the counting system to detect the beta particles emitted by the sample to be analysed.
When measuring sample activities with liquid scintillation counters, a basic problem is the reduction in counting efficiency due to the quenching in the sample. There are two types of quenching: chemical and color quenching. The chemical quenching is a phenomenon where the chemical impurities in the sample interfere with the reaction between the beta particle and the scintillator inhibiting the production of photons, and thus reducing the counting efficiency. Whereas, in the color quenching the impurities containing color absorb the photons of the scintillation, and thus reduce the counting efficiency.
Because the quenching reduces scintillation photons the spectrum also shits to lower channels of the multichannel analyser. Therefore, an appropriate numerical factor describing the position of the spectrum, e.g. the end point, maximum point or the center of the mass, can be used as a quenching parameter.
It is known that in liquid scintillation counting the reduction in the counting efficiency due to the quenching of the sample can be corrected by the use of a quench curve that describes the relationship between the counting efficiency and the amount of quench in the sample. The problem has been that the quench curves for chemically quenched and color quenched samples have not been exactly equal.
U.S. Pat. No. 4,700,072 describes a method where the difference in counting efficiencies in a color quenched sample compared to purely chemically quenched sample can be corrected using the fact that the color in the sample causes a change in the ratio of the pulse heights detected by the photo multiplier tubes working in coincidence. The reason for the change is that the scintillation photons travel through the colored solution of the samples longer to reach one of the photo multiplier tubes that then detects fewer photons that the other photo multiplier tube, because a portion of the photons are lost: they are absorbed into the solution during the travel to the photo multiplier tube.
The U.S. Pat. No. 5,061,853 describes a liquid scintillation counter that counts samples deposited on horizontally placed well plates. The counter measures the samples with a pair of photo multiplier tubes working in coincidence that are situated above and below the sample well plate. An example of a counter of this type is Wallac 1450 MicroBeta, manufactured by Wallac Oy, Finland. In this counter the center of the mass of the sum spectrum from a photo multiplier tube pair working in coincidence is used for determining the quench level of the sample.
Another liquid scintillation counter counting samples deposited on horizontally placed well-plates is presented in PCT Application No. 90114090.5 (EP 0 425 767 A1). This PCT application presents a liquid scintillation counter that measures samples deposited on well-plates with only a single photo multiplier tube sensing the scintillations from each sample. Instead of the coincidence method the background noise of the photo multiplier tubes is reduced by a counting system that discriminates between the pulses attributable to sample events and pulses attributable to non-sample events, such as photo multiplier tube noise, by determining whether a selected number of electrical pulses occur with a prescribed time interval.
A method where the difference in the counting efficiencies between the sample to be analysed and a sample containing purely chemical quench is determined is presented in Finnish Patent No. 86345. In this Finnish Patent the presented method calculates and corrects the difference in the counting efficiencies by measuring, before, or after, the actual liquid scintillation counting measurement, the quenching of a light pulse travelling through the sample with a photometric device placed in the liquid scintillation counter. The amount of quenching measured by the photometric device is used for correcting the quench in liquid scintillation measurement.
In the use of both of the above described methods there are some difficulties. Significant problems arise when measuring horizontally placed well-plates in a liquid scintillation counter of normal technical level with a coincidence method. There are also other sources of error besides the difference in color and chemically quenched samples, e.g. the vertical asymmetry of the sample to be counted. The vertical asymmetry in the sample is caused, e.g. when the scintillator particles with the sample to be analysed bound onto them are precipitated onto the bottom of the sample well.